1. Field of the Invention
This invention relates to a differential pressure sensor for measuring small pressure differences in an oil or gas well environment. Specifically, this environment will often include exposure to high temperatures, high pressures, corrosive media, shock and vibration. Also, an important additional requirement is usually a small diametrical size to fit within the flow bore of drill pipe, production tubing of casing.
2. Description of Related Art
Differential pressure measurements are useful in an oil or gas well to provide fluid head measurements such as for the determination of fluid density. This information is useful for quantifying the fluid composition being produced from or intruding into the well. Fluid composition identity and measurement has been successfully accomplished by radioactive means for many years. Regulations and required permits for radioactive substances, however, are becoming more cumbersome and difficult to obtain. The required red tape varies in different parts of the world and may be prohibitive. As a result of this issue, there is an increasing demand for a fluid densitometer that does not require radioactivity.
Another application for differential pressure measurement is quantifying the pressure drop developed across a flow measuring element, such as a venturi. This function can also be accomplished by using two precision static pressure gauges and taking the difference between their readings. This approach is limited for down hole applications, however, by the requirement of large full scale pressure ranges, since this pressure can be as much as 20,000 PSI, or more. The pressure differential across a flow element is normally less than 20 PSI. However, pressure gauge errors (inherent inaccuracy) are normally a percent of full scale. Consequently, measurement with a high pressure gauge may result in large errors. This gauge error may easily be 1000 times (20,000/20) greater than the stated accuracy of the individual gauges. This inaccuracy is further magnified by the square root relationship of the flow element differential pressure to flow.
Past attempts at differential pressure sensors, for this type of application, have met with many obstacles. First, the pressure difference to be measured is small, and difficult to measure in the presence of a high pressure environment. The high pressure environment also provides ample opportunity for over ranging and subsequent damage to the differential sensor. Secondly, the small diametrical size and wide operating temperature requirements, which are also coupled with potentially highly corrosive fluid media, greatly magnify the design difficulties.
It is an object of this invention, then, to provide a differential pressure sensor suitable for application in this environment.